Semiconductor device and method for fabricating the same

ABSTRACT

The semiconductor device comprises a gate insulating film including a first dielectric film of Hf x Al 1-x O y  (0.7&lt;x&lt;1) formed over a semiconductor substrate, and a second dielectric film different from the first dielectric film formed over the first dielectric film; and a gate electrode formed over the gate insulating film and including a polycrystalline silicon film, whereby the local abnormal growth of the polycrystalline silicon film in the process of forming the polycrystalline silicon film is prevented, and the gate leakage current can be much decreased.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 2003-029372, filed on Feb. 6,2003, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a semiconductor device and a method forfabricating the semiconductor device, more specifically to asemiconductor device including an MIS (Metal-Insulator-Semiconductor)transistor having a gate insulating film of a high dielectric constantfilm, and a method for fabricating the semiconductor device.

As MIS transistors are increasingly downsized due to higher integrationof semiconductor devices, the gate insulating films are madeincreasingly thinner. Transistors for gate lengths of below 50 nm areexpected several years later. Gate insulating films of film thicknessesof below 1 nm in terms of the thickness of the silicon oxide film arerequired.

As the gate insulating films, conventionally silicon oxide-basedinsulating films have been dominantly used. However, there has beennoted a problem that when the silicon oxide-based insulating films havethicknesses of below about 3 nm, the tunneling leakage current isconspicuous, and the silicon oxide-based insulating films fail tofunction as the insulating film. Studies are being made of forming thegate insulating films of new materials taking the place of the siliconoxide-based insulating films, whose thicknesses are below 1 nm in termsof the thickness of the silicon oxide-based insulating films.

It is being studied to use materials whose dielectric constants arehigher than the dielectric constant of silicon oxide (high-k materials)as a gate insulating film in place of the silicon oxide-based insulatingfilms. The use of the high-k materials permits the physical filmthickness of the gate insulating film to be thick, whereby the leakagecurrent can be suppressed.

As the high-k materials, various metal oxide materials are proposed;ZrO₂, Al₂O₃, HfO₂, TaO₂, etc. are noted. Among them, HfO₂ is prospectivebecause of advantages that the relative dielectric constant is about20˜30, which is high; HfO₂ is not easily silicidized more than ZrO₂;that the interfacial layer thereof with respect to a silicon substratedoes not much increase while being grown; etc.

However, as-grown HfO₂ is partially crystallized and disadvantageouslyhas large leakage current. It is known that when polycrystalline siliconfilm is grown on HfO₂, the polycrystalline silicon locally abnormallygrows.

As for the problem of the crystallization of HfO₂, it is proposed to mixa non-crystalline material in the gate insulating film as described in,e.g., Reference 1 (Japanese published unexamined patent application No.2001-267566). As described in Reference 2 (Japanese published unexaminedpatent application No. 2002-033320), it is proposed to mix SiO₂, Al₂O₃or others, which are not easily crystallized into HfO₂ to therebysuppress the crystallization so as to suppress the leakage current. Asfor the abnormal growth of the polycrystalline silicon, Reference 3 (D.C. Gilmer et al., “Compatibility of polycrystalline silicon gatedeposition with HfO₂ and Al₂O₃/HfO₂ gate dielectrics”, Appl. Phys. Lett.Vol. 81, pp. 1288-1290 (2002)) discloses that an AlO₃ film is formedbetween an HfO₂ film and a polycrystalline silicon film to therebysuppress the abnormal growth of the polycrystalline silicon.

SUMMARY OF THE INVENTION

The result of the earnest studies of the high-k films of HfO₂—Al₂O₃mixed system (hereinafter called Hf_(x)Al_(1-x)O_(y)) the inventors ofthe present application have made shows that when Al₂O₃, etc. are mixedin HfO₂ to thereby suppress the crystallization of the HfO₂, the gateleakage current is often increased in Hf-rich regions. It has been foundthat the polycrystalline silicon gate is abnormally grown to produceisland-shaped projections in the Hf-rich regions.

An object of the present invention is to provide a semiconductor deviceincluding MIS transistors having a gate insulating film containingHf_(x)Al_(1-x)O_(y), which can suppress the gate leakage current and theproduction of island-shaped projections, and a method for fabricatingthe semiconductor device.

According to one aspect of the present invention, there is provided asemiconductor device comprising: a gate insulating film including afirst dielectric film of Hf_(x)Al_(1-x)O_(y) in which x is 0.7<x<1,formed over a semiconductor substrate, and a second dielectric filmdifferent from the first dielectric film formed over the firstdielectric film; and a gate electrode formed on the gate insulating filmand including a polycrystalline silicon film.

According to another aspect of the present invention, there is provideda semiconductor device comprising: a gate insulating film formed on asemiconductor substrate and including an Hf_(x)Al_(1-x)O_(y) film havinga thickness below 1 nm in which x is 0.7<x<1; and a gate electrodeformed on the gate insulating film and including a polycrystallinesilicon film.

According to further another aspect of the present invention, there isprovided a method for fabricating a semiconductor device comprising thesteps of: forming over a semiconductor substrate a first dielectric filmof Hf_(x)Al_(1-x)O_(y) in which x is 0.7<x<1; forming a seconddielectric film different from the first dielectric film over the firstdielectric film; and forming a polycrystalline silicon film over thesecond dielectric film.

According to further another aspect of the present invention, there isprovided a method for fabricating a semiconductor device comprising thesteps of: forming a first dielectric film of a silicon-based insulatingfilm over a semiconductor substrate; forming over the first dielectricfilm a second dielectric film of Hf_(x)Al_(1-x)O_(y) having a thicknessbelow 1 nm in which x is 0.7<x<1; and forming a polycrystalline siliconfilm over the second dielectric film.

According to further another aspect of the present invention, there isprovided a method for fabricating a semiconductor device comprising thesteps of: forming a dielectric film of Hf_(x)Al_(1-x)O_(y) over asemiconductor substrate; and forming over the dielectric film a siliconfilm at a temperature of below 550° C.

According to the present invention, between an Hf_(x)Al_(1-x)O_(y) filmand a silicon film, an insulating film which can suppress the abnormalgrowth of the silicon film, the thickness of the Hf_(x)Al_(1-x)O_(y)film is set to be below 1 nm, or the silicon film is formed in amorphousstate on Hf_(x)Al_(1-x)O_(y), whereby the local abnormal growth of thesilicon film in forming the silicon film on the Hf_(x)Al_(1-x)O_(y) canbe suppressed. The gate leakage current can be much decreased.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1C, 1E, and 1G are views showing surface states of thepolycrystalline silicon films.

FIGS. 1B, 1D, 1F, and 1H are views showing intra-plane leakage currents.

FIG. 2 is a graph showing hafnium composition and gate area dependencyof the gate leakage current characteristics.

FIGS. 3A-3E are topographic images of surface states of thepolycrystalline silicon films, which change with the hafnium compositionchanged.

FIG. 4 is a graph showing relationships between the number and theheight of island-shaped projections.

FIG. 5 is a topographic image of the surface state of thepolycrystalline silicon film with the silicon nitride film formedbetween the Hf_(0.8)Al_(0.2)O_(y) film and the polycrystalline siliconfilm.

FIG. 6 is a graph showing gate area dependency of the gate leakagecurrent with a silicon nitride film formed between theHf_(0.8)Al_(0.2)O_(y) film and the polycrystalline silicon film.

FIGS. 7A-7C are topographic images of surface states of thepolycrystalline silicon films, which change with the film thickness ofthe Hf_(0.8)Al_(0.2)O_(y) film changed.

FIG. 8 is a topographic image of the surface state of the amorphoussilicon deposited on the Hf_(0.8)Al_(0.2)O_(y).

FIG. 9 is a diagrammatic sectional view of the semiconductor deviceaccording to a first embodiment of the present invention, which shows astructure thereof.

FIGS. 10A-10D and 11A-11C are sectional views of the semiconductordevice according to the first embodiment of the present invention in thesteps of the method for fabricating the same, which show the method.

FIG. 12 is a diagrammatic sectional view of the semiconductor deviceaccording to a second embodiment of the present invention, which shows astructure thereof.

FIG. 13 is a diagrammatic sectional view of the semiconductor deviceaccording to a third embodiment of the present invention, which shows astructure thereof.

FIG. 14 is a diagrammatic sectional view of the semiconductor deviceaccording to a fourth embodiment of the present invention, which shows astructure thereof.

FIGS. 15A-15C are sectional views of the semiconductor device accordingto the fourth embodiment of the present invention in the steps of themethod for fabricating the same, which show the method.

FIG. 16 is a diagrammatic sectional view of the semiconductor deviceaccording to a fifth embodiment of the present invention, which shows astructure thereof.

FIGS. 17A-17C are sectional views of the semiconductor device accordingto the fifth embodiment of the present invention in the steps of themethod for fabricating the same, which show the method.

DETAILED DESCRIPTION OF THE INVENTION Principle According to theEmbodiments of the Present Invention

The result of the earnest studies of Hf_(x)Al_(1-x)O_(y) the inventorsof the present application have made shows that the leakage currentthrough Hf_(x)Al_(1-x)O_(y) film is much dependent on the hafniumcomposition (x). When the hafnium composition x is above 0.9, largeleakage current is observed. This will be because theHf_(x)Al_(1-x)O_(y) film even as-deposited state is partiallycrystallized. When polycrystalline silicon film to form gate electrodesis formed on the Hf_(x)Al_(1-x)O_(y) film with an Hf composition x of0.7<x<1, the polycrystalline silicon film is locally abnormally grown,and large island-shaped projections are produced. It is not knownwhether the abnormal growth is related directly to the leakage current,but the produced island-shaped projections affect the semiconductordevice fabrication process, and the semiconductor device cannot befabricated as designed.

To scrutinize these phenomena, the inventors of the present applicationinvestigated local leakage current characteristics of thepolycrystalline silicon/Hf_(x)Al_(1-x)O_(y) structure with AFM (AtomicForce Microscope). FIGS. 1A-1H show the results. FIG. 1A is a surfacetopographic image with an Hf composition of x=0.7. FIG. 1B is a view ofthe intra-plane leakage current observed simultaneously with taking thetopographic image of FIG. 1A. FIG. 1C is a surface topographic imagewith an Hf composition of x=0.8. FIG. 1D is a view of the intra-planeleakage current observed simultaneously with taking the topographicimage of FIG. 1C. FIG. 1E is a surface topographic image with an Hfcomposition of x=0.9. FIG. 1F is a view of the intra-plane leakagecurrent observed simultaneously with taking the topographic image ofFIG. 1E. FIG. 1G is a surface topographic image with an Hf compositionof x=1.0. FIG. 1H is a view of the intra-plane leakage current observedsimultaneously with taking the topographic image of FIG. 1G.

In the topographic images of FIGS. 1C and 1E, a number of island-shapedprojections are observed. These island-shaped projections were producedby the abnormal growth of the polycrystalline silicon. On the otherhand, with hafnium compositions of x=1.0 and x=0.7, as shown in FIGS. 1Aand 1G, no island-shaped projections have been produced. It has beenfound that the island-shaped projections are not observed at the earlystage of the growth of the polycrystalline silicon film but are producedwhen the polycrystalline silicon film has grown into a sufficientthickness.

On the other hand, as shown in FIGS. 1B, 1D, 1F and 1H, it has beenfound that more leakage current takes place as the hafnium composition xis larger. The spots of the leakage current do no always agree with thepositions of the island-shaped projections. It has been found that witha hafnium composition of x=0.8, leaky spots are locally present. Theselocal leakage current spots take place at a 1-2 spots per 4 μm² ratio,which corresponds to the densities of the island-shaped projectionsproduced.

FIG. 2 is a graph of the hafnium composition and gate area dependency ofthe gate leakage current of MOS capacitors having the gate insulatingfilms of Hf_(x)Al_(1-x)O_(y). As shown in FIG. 2, in the samples havingthe hafnium compositions of x=0.5 and x=1.0, the variations of theleakage current are not large even as the gate areas are decreased. Inthe sample having the hafnium composition of x=0.8, the variations ofthe leakage current are larger as the gate areas are decreased. Thisphenomena will be that when the gate area is large, a number of localleakage spots contained in the gate area is evened to thereby make thevariations small, while when the gate area is small, and local leakagecurrent spots are present in the gate, the leakage current is small whenthe local leakage current spots are not leaky.

FIGS. 3A-3E are topographic images explaining relationships between thehafnium composition and the abnormal growth of the polycrystallinesilicon. In FIG. 3A, the hafnium composition x is 0.6. In FIG. 3B, thehafnium composition x is 0.7. In FIG. 3C, the hafnium composition x is0.8. In FIG. 3D, the hafnium composition x is 0.9. In FIG. 3E, thehafnium composition x is 1.0.

As shown in FIGS. 3A-3E, when the hafnium composition x is 0.7 or less(0<x≦0.7) and x=1.0, no local abnormal growth of the polycrystallinesilicon is observed, but the local abnormal growth of thepolycrystalline silicon is grown when the hafnium composition x is 0.8and 0.9.

FIG. 4 is a graph of relationships between the number and the height ofthe precipitations. In FIG. 4, the ● marks indicate the case of thehafnium composition x=1.0; the ▪ marks indicate the case of the hafniumcomposition x=0.9; the ▾ marks indicate the case of the hafniumcomposition x=0.8; the ∘ marks indicate the case of the hafniumcomposition x=0.7; and the □ marks indicate the case of the hafniumcomposition x=0.6.

As shown, in the cases of the hafnium composition x=0.8 and the hafniumcomposition x=0.9, the heights of the precipitations are distributedhigh. The abnormal growth is found.

The hafnium composition x=1.0 corresponds to HfO₂ and is the conditionfor the observed abnormal growth of the polycrystalline silicon in theabove-described Reference 3. However, the inventors of the presentapplication have not confirmed the abnormal growth, based on the resultof their studies. The result of the studies of the inventors of thepresent application shows that the abnormal growth takes place morefrequently in the case of the hafnium composition x=0.9 than in the caseof the hafnium composition x=0.8. Based on this, the abnormal growth ofthe polycrystalline silicon in the cases of the hafnium compositionsx=0.8 and x=0.9 is different from the abnormal growth of thepolycrystalline silicon found in the Reference 3 and will be a phenomenabased on the specific characteristic of Hf_(x)Al_(1-x)O_(y).

As described above, when Hf_(x)Al_(1-x)O_(y) has the hafnium compositionx of 0.7<x<1, the abnormal growth of the polycrystalline silicon takesplace. Accordingly, to prevent the abnormal growth of thepolycrystalline silicon, the hafnium composition x is set to be below0.7. However, when the hafnium composition x is below 0.5, the effect ofthe leakage current can be produced, but the relative dielectricconstant is lower for the large Al₂O₃ composition. To obtain highdielectric constant, it is preferable to form a film having a higherHfO₂ composition.

Then, first means of the present invention is to form an insulating filmbetween the Hf_(x)Al_(1-x)O_(y) film and the polycrystalline siliconfilm, which can suppress the abnormal growth of the polycrystallinesilicon film. The abnormal growth of the polycrystalline silicon is dueto forming the polycrystalline silicon film directly on theHf_(x)Al_(1-x)O_(y) film of the above-described composition.Accordingly, the abnormal growth of the polycrystalline silicon film canbe suppressed by interposing an insulating film, such as silicon oxidefilm, silicon nitride film, silicon oxynitride film, Al₂O₃ (alumina)film, film having a HfO₂ composition x of below 0.7 including 0.7(0<x≦0.7) or others, which can suppress the abnormal growth of thepolycrystalline silicon.

FIG. 5 is a topographic image of a silicon nitride film formed betweenthe Hf_(0.8)Al_(0.2)O_(y) film and the polycrystalline silicon film. Asevident in the comparison between FIG. 3C and FIG. 5, the siliconnitride film is interposed, whereby the local abnormal growth of thepolycrystalline silicon can be suppressed.

In FIG. 4, the ∇ marks indicate the relationship between the number andthe height of the precipitations when the silicon nitride film is formedbetween the Hf_(0.8)Al_(0.2)O_(y) film and the polycrystalline siliconfilm. It is evident also from the graph of FIG. 4 that the formation ofthe silicon nitride film can suppress the abnormal growth of thepolycrystalline silicon.

FIG. 6 is a graph of the gate area dependency of the gate leakagecurrent in the case that the silicon nitride film is formed between theHf_(0.8)Al_(0.2)O_(y) film and the polycrystalline silicon film. Asshown, the presence of the silicon nitride film can more decrease theleakage current and makes variations of the leakage current small incomparison with the leakage current in the case that the silicon nitridefilm is not formed.

In second means of the present invention, the film thickness ofHf_(x)Al_(1-x)O_(y) is set to be below 1 nm. When the film thickness ofHf_(x)Al_(1-x)O_(y) is below 1 nm, even with the hafnium composition xbeing 0.7<x<1, the abnormal growth of the polycrystalline silicon can besuppressed. In this case, however, it is preferable to form the gateinsulating film of a stacked film of other insulating films so as toensure a sufficient physical film thickness as the gate insulating film.

FIGS. 7A-7C are views of surface states of the Hf_(0.8)Al_(0.2)O_(y)films with different film thicknesses. FIG. 7A shows the surface stateof the Hf_(0.8)Al_(0.2)O_(y) film of a 2 nm-thick. FIG. 7B is thesurface state of the Hf_(0.8)Al_(0.2)O_(y) film of a 1 nm-thick. FIG. 7Cis the surface state of the Hf_(0.8)Al_(0.2)O_(y) film of a 0.5nm-thick.

As shown, when the film thickness is below 1 nm, the island-shapedprojections produced by the abnormal growth of the polycrystallinesilicon can be much suppressed.

In third means of the present invention, amorphous silicon is depositedin place of depositing polycrystalline silicon on Hf_(x)Al_(1-x)O_(y).The earnest studies of the inventors of the present invention has foundthat when the film forming temperature is lowered to the film formingtemperature of the amorphous silicon, the size of the island-shapedprojections produced by the abnormal growth become smaller. Thus,amorphous silicon film is deposited in place of polycrystalline siliconfilm, whereby the abnormal growth can be suppressed.

FIG. 8 is a topographic view of amorphous silicon deposited on theHf_(0.8)Al_(0.2)O_(y) at 550° C. As evident from the comparison betweenFIG. 3C and FIG. 8, the film forming temperature is made low, wherebysizes of the island-shaped projections can be made small.

The x marks in FIG. 4 indicate the number and height of depositions whenamorphous silicon film is deposited on the Hf_(x)Al_(1-x)O_(y) film. Itis evident from FIG. 4 that the amorphous silicon film is deposited tothereby make the island-shaped projections small.

Hf_(x)Al_(1-x)O_(y) is correctly expressed by (HfO₂)_(x)(Al₂O₃).However, the respective oxidized states of the Hf and Al are notspecifically defined, and the oxygen composition y varies depending onthe hafnium composition x.

A First Embodiment

The semiconductor device and the method for fabricating the sameaccording to a first embodiment of the present invention will beexplained with reference to FIGS. 9 to 11C.

FIG. 9 is a sectional view of the semiconductor device according to thepresent embodiment, which shows a structure thereof. FIGS. 10A-10D and11A-11C are sectional views of the semiconductor device according to thepresent embodiment in the steps of the method for fabricating the same,which show the method.

First, the structure of the semiconductor device according to thepresent embodiment will be explained with reference to FIG. 9.

A device isolation film 12 is formed on a silicon substrate 10. A gateinsulating film 20 including an interfacial layer 14, anHf_(0.8)Al_(0.2)O_(y) film 16 and an Al₂O₃ (alumina) film 18 is formedon the silicon substrate in the device region defined by the deviceisolation film 12. A gate electrode 24 of a polycrystalline silicon filmis formed on the gate insulating film 20. A sidewall insulating film 28is formed on the sidewalls of the gate electrode 24. Source/draindiffused layers 32 are formed in the silicon substrate 10 on both sidesof the gate electrode 24.

As described above, the semiconductor device according to the presentembodiment is characterized mainly in that the gate insulating film 20includes the Al₂O₃ film 18 formed on the Hf_(0.8)Al_(0.2)O_(y) film 16.As described above, the Hf_(x)Al_(1-x)O_(y) film having the hafniumcomposition x of 0.7<x<1 is deposited directly on the polycrystallinesilicon film, the local abnormal growth of the polycrystalline silicontakes place, which results in increase of the gate leakage current. Asin the semiconductor device according to the present embodiment, theAl₂O₃ film 18 is formed on the Hf_(0.8)Al_(0.2)O_(y) film 16, and thepolycrystalline silicon film is deposited on the Al₂O₃ film 18, wherebythe abnormal growth of the polycrystalline silicon can be prevented.Accordingly, the semiconductor device according to the presentembodiment can decrease the gate leakage current.

Next, the method for fabricating the semiconductor device according tothe present embodiment will be explained with reference to FIGS. 10A to11C.

First, a device isolation film 12 is formed on the silicon substrate 10by, e.g., STI (Shallow Trench Isolation) method (FIG. 10A).

Next, the Hf_(0.8)Al_(0.2)O_(y) film 16 of a 3 nm-thick is deposited byMOCVD method on the silicon substrate 10 with the device isolation film12 formed on. The Hf_(0.8)Al_(0.2)O_(y) film 16 is formed by, e.g., byusing TTBAl (tri-tertiary butyl Al) as the Al raw material, TTBHf (tetrabutoxy Hf) as the Hf raw material, O₂ gas as the oxidation gas and N₂gas as the carrier gas, at a 500° C. substrate temperature, a 300 sccmTTBAl flow rate, a 35 sccm TTBHf flow rate, a 100 sccm O₂ flow rate anda 1500 sccm total flow rate.

When the Hf_(0.8)Al_(0.2)O_(y) film 16 is formed, an interfacial layer14 is formed between the silicon substrate 10 and theHf_(0.8)Al_(0.2)O_(y) film 16. The interfacial layer will be formed bythe silicon substrate 10 being oxidized in forming theHf_(0.8)Al_(0.2)O_(y) film 16.

Next, the Al₂O₃ film 18 of, e.g., a 1 nm-thick is formed on theHf_(0.8)Al_(0.2)O_(y) film 16 (FIG. 10B). The Al₂O₃ film 18 is formedby, e.g., using TTBAl as the Al raw material, O₂ gas as the oxidizinggas and N₂ gas as the carrier gas, and at a 500° C. substratetemperature, a 300 sccm TTBAl flow rate, a 100 sccm O₂ flow rate and a1500 sccm total flow rate.

It is preferable that the Hf_(0.8)Al_(0.2)O_(y) film 16 is continuouslyformed in the same film forming chamber as the Hf_(0.8)Al_(0.2)O_(y)film 16 has been deposited. The Al₂O₃ film 18 can be deposited bystopping the supply a hafnium source used in depositing theHf_(0.8)Al_(0.2)O_(y) film 16. This prevents the Hf_(0.8)Al_(0.2)O_(y)film 16 from being exposed to the ambient atmosphere, and the interfacecan have good quality.

Thus, the gate insulating film 20 of the stacked film of theHf_(0.8)Al_(0.2)O_(y) film 16 and the Al₂O₃ film 18 can be formed on thesilicon substrate 10.

Next, the polycrystalline silicon film 22 of, e.g., a 150 nm-thick isformed on the gate insulating film 20 by, e.g., low-pressure CVD method(FIG. 10C). The polycrystalline silicon film 22 is formed, e.g., byusing SiH₄ (20%) and He (80%) as the raw materials, and at a 500 sccmtotal flow rate, a 30 Pa chamber pressure and a 620° C. film formingtemperature.

The above-described conditions for forming the polycrystalline siliconfilm 22 produce the local abnormal growth when the polycrystallinesilicon film is formed directly on the Hf_(0.8)Al_(0.2)O_(y) film 16. Inthe present embodiment, however, because of the Al₂O₃ film 18 formedbetween the Hf_(0.8)Al_(0.2)O_(y) film 16 and the polycrystallinesilicon film 22, the local abnormal growth of the polycrystallinesilicon film 22 does not take place.

Then, the polycrystalline silicon film 22 is patterned byphotolithography and dry etching to form the gate electrode 24 of thepolycrystalline silicon film 22 (FIG. 10D).

Next, with the gate electrode 24 as the mask, arsenic ions, for example,are implanted to form impurity diffused regions 26 to be LDD regions orextension regions in the silicon substrate 10 on both sides of the gateelectrode 24 (FIG. 11A).

Then, a silicon nitride film of, e.g., a 110 nm-thick is deposited by,e.g., CVD method, and the silicon nitride film is etched back to formthe sidewall insulating film 28 of the silicon nitride film on the sidewalls of the gate electrode 24 (FIG. 11B).

Next, with the gate electrode 24 and the sidewall insulating film 28 asthe mask, arsenic ions, for example, are implanted to form impuritydiffused regions 30 in the silicon substrate 10 on both sides of thegate electrode 24.

Then, the implanted impurities are activated by the rapid thermalprocessing of, e.g., 1500° C. and 1 second to form the source/draindiffused layer 32 of the impurity diffused regions 26, 30 (FIG. 1C).

Thus, the semiconductor device shown in FIG. 9 is fabricated.

As described above, according to the present embodiment, the Al₂O₃ filmis formed between the Hf_(0.8)Al_(0.2)O_(y) film and the polycrystallinesilicon film, whereby the local abnormal growth of the polycrystallinesilicon film in the process of forming the polycrystalline silicon filmcan be prevented. The gate leakage current can be much decreased.

A Second Embodiment

The semiconductor device and the method for fabricating the sameaccording to a second embodiment of the present invention will beexplained with reference to FIG. 12. The same members of the presentembodiment as those of the semiconductor device and the method forfabricating the same according to the first embodiment shown in FIGS. 9to 11C are represented by the same reference numbers not to repeat or tosimplify their explanation.

FIG. 12 is a diagrammatic sectional view of the semiconductor deviceaccording to the present embodiment, which shows a structure thereof.

The semiconductor device according to the present embodiment is the samein the basic structure as the semiconductor device according to thefirst embodiment shown in FIG. 9. The semiconductor device according tothe present embodiment is different from the semiconductor deviceaccording to the first embodiment in that in the former, a siliconnitride film 34 is formed in place of the Al₂O₃ film 18.

The silicon nitride film 34 has the effect of preventing the localabnormal growth of polycrystalline silicon, as does the Al₂O₃ film 18.Accordingly, the silicon nitride film 34 is formed between theHf_(0.8)Al_(0.2)O_(y) film 16 and the gate electrode 24, whereby theabnormal growth of polycrystalline silicon can be prevented.Accordingly, the semiconductor device according to the presentembodiment can decrease the gate leakage current.

Nitrogen-content silicon-based insulating films have the effect ofsuppressing the diffusion of boron. Accordingly, in p-channeltransistors, the diffusion of boron from the gate electrode 24 to thesilicon substrate 10 can be prevented, whereby degradation of thetransistor characteristics due to the diffusion of boron can beprevented.

The semiconductor device according to the present embodiment can befabricated by the method for fabricating the semiconductor deviceaccording to the first embodiment, in which the silicon nitride film 34is formed in place of the Al₂O₃ film 18.

The silicon nitride film 34 can be formed by depositing a siliconnitride film of, e.g., a 0.5 nm-thick by, e.g., low-pressure CVD method.It is preferable that the silicon nitride film, whose relativedielectric constant is lower than alumina, is formed thinner thanalumina film.

As described above, according to the present embodiment, the siliconnitride film is formed between the Hf_(0.8)Al_(0.2)O_(y) film and thepolycrystalline silicon film, whereby the local abnormal growth of thepolycrystalline silicon film in the process of forming thepolycrystalline silicon film can be prevented. The gate leakage currentcan be much decreased.

In the present embodiment, the silicon nitride film is formed on theHf_(0.8)Al_(0.2)O_(y) film, but SiON (silicon oxynitride) film may beformed in place of the silicon nitride film.

A Third Embodiment

The semiconductor device and the method for fabricating the sameaccording to a third embodiment of the present invention will beexplained with reference to FIG. 13. The same members of the presentembodiment as those of the semiconductor device and the method forfabricating the same according to the first and the second embodimentsshown in FIGS. 9 to 12 are represented by the same reference numbers notto repeat or to simplify their explanation.

FIG. 13 is a diagrammatic sectional view of the semiconductor deviceaccording to the present embodiment, which shows a structure thereof.

The semiconductor device according to the present embodiment is the samein the basic structure as the semiconductor device according to thefirst embodiment shown in FIG. 9. The semiconductor device according tothe present embodiment is different from the semiconductor deviceaccording to the first embodiment in that in the former anHf_(0.5)Al_(0.5)O_(y) film 36 is formed in place of the Al₂O₃ film 18.

When Hf_(x)Al_(1-x)O_(y) has a hafnium composition x of below 0.7, asdescribed above, the local abnormal growth does not take place even whenthe polycrystalline silicon is deposited directly on theHf_(x)Al_(1-x)O_(y) film. Accordingly, the hafnium composition of atleast the uppermost part of the Hf_(x)Al_(1-x)O_(y), which contacts thepolycrystalline silicon has the hafnium composition x of below 0.7,whereby the abnormal growth of the polycrystalline silicon can beprevented. Accordingly, the semiconductor device according to thepresent embodiment can decrease the gate leakage current.

The semiconductor device according to the present embodiment can befabricated by the method for fabricating the semiconductor deviceaccording to the first embodiment, in which the Hf_(0.5)Al_(0.5)O_(y)film is formed in place of the Al₂O₃ 18. Specifically, theHf_(0.5)Al_(0.5)O_(y) film is formed, e.g., by using TTBAl as the Al rawmaterial, TTBHf as the Hf raw material, O₂ gas as the oxidation gas andN₂ gas as the carrier gas, and at a 500° C. substrate temperature, a 500sccm flow rate of the TTBHf, a 140 sccm flow rate of the TTBAl, a 100sccm flow rate of O₂ and a 1500 sccm total flow rate. Thus, theHf_(0.5)Al_(0.5)O_(y) film 36 can be formed. The film thickness of theHf_(0.5)Al_(0.5)O_(y) film 36 is set to, e.g., 1 nm.

The Hf_(0.5)Al_(0.5)O_(y) film, whose relative dielectric constant ishigher than the relative dielectric constants of Al₂O₃ and siliconnitride film, advantageously allows the physical film thickness of thegate insulating film 20 larger than the gage insulating film of thesemiconductor device according to the first and the second embodiments.To obtain the relative dielectric constant higher, it is preferable toapproximate the hafnium composition x to 0.7 which is upper limit.

As described above, according to the present embodiment, theHf_(x)Al_(1-x)O_(y) film having a below 0.7 hafnium composition x isformed between the Hf_(x)Al_(1-x)O_(y) film and the polycrystallinesilicon film, whereby the local abnormal growth of the polycrystallinesilicon can be prevented. The presence of the material whose relativedielectric constant is higher than the relative dielectric constants ofthe semiconductor device according to the first and the secondembodiments allows the gate insulating film to have a larger physicalfilm thickness. Accordingly, the gate leakage current can be muchdecreased.

In the present embodiment, the Hf_(0.5)Al_(0.5)O_(y) film 36 is formedon the Hf_(0.8)Al_(0.2)O_(y) film 16. However, the hafnium composition xof the Hf_(x)Al_(1-x)O_(y) at least at the uppermost part which contactsthe polycrystalline silicon is below 0.7, whereby the abnormal growth ofthe polycrystalline silicon can be prevented. Accordingly, the hafniumcomposition of the Hf_(0.5)Al_(0.5)O_(y) film 36 is not limited to thiscomposition. The hafnium composition is not essentially changed in astep, and the Hf_(x)Al_(1-x)O_(y) film can be a composition graded layerwhose hafnium composition is gradually decreased to a below 0.7 hafniumcomposition x at the surface.

A Fourth Embodiment

The semiconductor device and the method for fabricating the sameaccording to a fourth embodiment of the present invention will beexplained with reference to FIGS. 14 and 15A-15C. The same members ofthe present embodiment as those of the semiconductor device and themethod for fabricating the same according to the first to the thirdembodiments shown in FIGS. 9 to 13 are represented by the same referencenumbers not to repeat or to simplify their explanation.

FIG. 14 is a diagrammatic sectional view of the semiconductor deviceaccording to the present embodiment, which shows a structure thereof.FIGS. 15A-15C are sectional views of the semiconductor device accordingto the present embodiment in the steps of the method for fabricating thesame, which show the method.

First, the structure of the semiconductor device according to thepresent embodiment will be explained with reference to FIG. 14.

A device isolation film 12 is formed on a silicon substrate 10. A gateinsulating film 20 of a 1 nm-thick SiON film 38 and a 1 nm-thickHf_(0.8)Al_(0.2)O_(y) film 16 is formed on a silicon substrate 10 in thedevice region defined by the device isolation film 12. A gate electrode24 of a polycrystalline silicon film is formed on the gate insulatingfilm 20. A side wall insulating film 28 is formed on the side walls ofthe gate electrode 24. Source/drain diffused layers 32 are formed in thesilicon substrate 10 on both sides of the gate electrode 24.

As described above, the semiconductor device according to the presentembodiment is characterized mainly in that the gate insulating film 20is formed of a 1 nm-thick SiON film 38 and a 1 nm-thickHf_(0.8)Al_(0.2)O_(y) film 16. As described above, the film thickness ofthe Hf_(x)Al_(1-x)O_(y) is set to be below 1 nm, whereby the localabnormal growth of the polycrystalline silicon can be suppressed. TheSiON film 38 is formed below the Hf_(0.8)Al_(0.2)O_(y) film 16 so as toincrease the physical film thickness of the gate insulating film 20.Thus, the semiconductor device according to the present embodiment candecrease the gate leakage current. In a p-channel transistor includingthe gate electrode 24 doped with boron, the use of the insulating filmof nitrogen-content silicon oxide can prevent the diffusion of boronfrom the gate electrode 24 to the silicon substrate 10. Accordingly,degradation of the transistor characteristics due to the diffusion ofboron can be prevented.

Next, the method for fabricating the semiconductor device according tothe present embodiment will be explained with reference to FIGS.15A-15C.

First, the device isolation film 12 for defining the device region isformed on the silicon substrate 10 by, e.g., STI method.

Next, the SiON film 38 of, e.g., a 1 nm-thick is formed by thermaloxidation on the silicon substrate 10 with the device isolation film 12formed on (FIG. 15A).

Then, the Hf_(0.8)Al_(0.2)O_(y) film 16 of a 1 nm-thick is deposited onthe SiON film 38 by, e.g., MOCVD method (FIG. 15B).

Next, the polycrystalline silicon film 22 of, e.g., a 150 nm-thick isformed on the gate insulating film 20 by, e.g., low-pressure CVD method(FIG. 15C).

The above-described conditions for forming the polycrystalline siliconfilm 22 produce the local abnormal growth when the polycrystallinesilicon film is deposited directly on the Hf_(0.8)Al_(0.2)O_(y) film 16.In the present embodiment, however, the film thickness of theHf_(0.8)Al_(0.2)O_(y) film 16 is 1 nm, which suppresses the localabnormal growth of the polycrystalline silicon film 22.

Then, in the same way as in, e.g., the method for fabricating thesemiconductor device according to the first embodiment shown in FIGS.10D to 11C, the gate electrode 24, the source/drain diffused layers 32,etc. are formed.

As described above, according to the present embodiment, the filmthickness of the Hf_(x)Al_(1-x)O_(y) is below 1 nm, whereby the localabnormal growth of the polycrystalline silicon can be suppressed. TheSiON film is formed below the Hf_(x)Al_(1-x)O_(y) film, whereby thephysical film thickness of the gate insulating film is allowed to beincreased, and in a p-channel transistor, the diffusion of boron fromthe gate electrode to the substrate can be prevented. Thus, thesemiconductor device according to the present embodiment can decreasethe gate leakage current.

A Fifth Embodiment

The semiconductor device and the method for fabricating the sameaccording to a fifth embodiment of the present invention will beexplained with reference to FIGS. 16 and 17A-17C. The same members ofthe present embodiment as those of the semiconductor device and themethod for fabricating the same according to the first to the fourthembodiments shown in FIGS. 9 to 15C are represented by the samereference numbers not to repeat or to simplify their explanation.

FIG. 16 is a diagrammatic sectional view of the semiconductor deviceaccording to the present embodiment, which shows a structure thereof.FIGS. 17A-17C are sectional views of the semiconductor device accordingto the present embodiment in the steps of the method for fabricating thesame, which show the method.

First, the structure of the semiconductor device according to thepresent embodiment will be explained with reference to FIG. 16.

A device isolation film 12 is formed on a silicon substrate 10. A gateinsulating film 20 including an interfacial layer 14 and anHf_(0.8)Al_(0.2)O_(y) film 16 is formed on the silicon substrate 10 in adevice region defined by the device isolation film 12. A gate electrode24 of a polycrystalline silicon film is formed on the gate insulatingfilm 20. A sidewall insulating film 28 is formed on the side walls ofthe gate electrode 24. Source/drain diffused layers 32 are formed in thesilicon substrate 10 on both sides of the gate electrode 24.

Next, the method for fabricating the semiconductor device according tothe present embodiment will be explained with reference to FIG. 17.

First, the device isolation film 12 for defining the device region isformed on the silicon substrate 10 by, e.g., STI method (FIG. 17A).

Then, the Hf_(0.8)Al_(0.2)O_(y) film 16 of a 3 nm-thick is formed byMOCVD method on the silicon substrate 10 with the device isolation film12 formed on (FIG. 17B).

When the Hf_(0.8)Al_(0.2)O_(y) film 16 is formed, the interfacial layer14 is formed in the interface between the silicon substrate 10 and theHf_(0.8)Al_(0.2)O_(y) film 16.

Next, an amorphous silicon film 40 of, e.g., a 150 nm-thick is formed onthe gate insulating film 20 by, e.g., low-pressure CVD method (FIG.17C). The amorphous silicon film 40 is formed, e.g., by using SiH₄ (20%)and He (80%) as the raw materials at a 500 sccm total flow rate, a 30 Pachamber pressure and a 550° C. film forming temperature. Decreasing thefilm forming temperature to 550° C. or below allows the amorphoussilicon film to be deposited under the same film forming conditions asthe polycrystalline silicon film.

Decreasing the temperature for forming the film forming the gateelectrode to the temperature for forming the amorphous silicon cansuppress the local abnormal growth in the process of forming the film.

Then, in the same way as in, e.g., the method for fabricating thesemiconductor device according to the first embodiment shown in FIGS.10D to 11D, the gate electrode 24, the source/drain diffused layers 32,etc. are formed.

The amorphous silicon film 40 is crystallized into polycrystallinesilicon when the impurities are activated by thermal processing.However, no island-shaped projections are abnormally grown in thethermal processing.

As described above, according to the present embodiment, amorphoussilicon is deposited on Hf_(x)Al_(1-x)O_(y), whereby the abnormal growthof the island-shaped projections can be prevented in the film formingprocess and the following crystallization thermal processing.

[Modifications]

The present invention is not limited to the above-described embodimentsand can cover other various modifications.

For example, in the first to the third embodiments described above, thenitrogen-content silicon-based insulating film, the alumina film or theHf_(x)Al_(1-x)O_(y) film of a hafnium composition x=0.7 are formed on anHf_(x)Al_(1-x)O_(y) film, but any dielectric film can be used as long asthe dielectric film is capable of suppressing the abnormal growth of thepolycrystalline silicon film. The dielectric film on theHf_(x)Al_(1-x)O_(y) film of a hafnium composition x of 0.7<x<1 is notessentially 1 layer, and 2 or more layers can be formed.

In the fourth embodiment, the SiON film is formed between the siliconsubstrate and the Hf_(x)Al_(1-x)O_(y) film, but in place of the SiONfilm, another dielectric film may be formed. For example, a siliconoxide film or a silicon nitride film can be used in place of the SiONfilm. From the viewpoint of preventing the diffusion of boron from thepolycrystalline silicon film to the substrate, it is preferable to usenitrogen-content silicon-based insulating film.

In the first to the fifth embodiments, the present invention is appliedto semiconductor devices having the gate electrodes formed ofpolycrystalline silicon film, but the structures of the gate electrodesare not limited to the above. For example, the present invention isapplicable to semiconductor devices of the polycide gate structure ofthe stacked film of a polycrystalline silicon film and a silicide filmor the polymetal gate structure of the stacked film of a polycrystallinesilicon film and a metal film.

The present invention is applicable to the semiconductor devicesincluding gate electrodes formed by forming dummy gate electrodes ofpolycrystalline silicon film on the gate insulating film and replacingthe dummy gate electrodes by metal materials such as aluminum or others.

In the above-described embodiments, the present invention is applied ton-channel MIS transistors but is applicable to p-channel MIStransistors.

1. A semiconductor device comprising: a gate insulating film including afirst dielectric film of Hf_(x)Al_(1-x)O_(y) in which x is 0.7<x<1,formed over a semiconductor substrate, and a second dielectric film of anitrogen-content silicon-based insulating film, an alumina film or anHf_(x)Al_(1-x)O_(y) film in which X is 0<x<0.7 formed over the firstdielectric film; and a gate electrode formed on the gate insulating filmand including a polycrystalline silicon film.
 2. A semiconductor devicecomprising: a gate insulating film formed on a semiconductor substrateand including an Hf_(x)Al_(1-x)O_(y) film having a thickness below 1 nmin which x is 0.7<x<1; and a gate electrode formed on the gateinsulating film and including a polycrystalline silicon film.
 3. Asemiconductor device according to claim 2, wherein the gate insulatingfilm further including a nitrogen-content silicon-based insulating filmformed between the semiconductor substrate and the Hf_(x)Al_(1-x)O_(y)film.
 4. A semiconductor device according to claim 3, wherein thenitrogen-content silicon-based insulating film is a silicon nitride filmor a silicon oxynitride film.